Digital oscilloscopes as shown in FIG. 1 consist of an input amplifier, of which the variable amplification factor is used to match the amplitude of the measured signal to be displayed on the oscilloscope with the measurement range of the oscilloscope; a downstream analog-digital converter for generating sampled values of the digitized measured signal; a recording unit for presenting over time the sampled values of the digitized measured signal; and a triggering system connected in parallel to the analog-digital converter for phase-corrected or respectively time-corrected presentation in the recording unit of the signal portion of the measured signal identified with the triggering threshold of the analog triggering system.
Via several comparators, an analog triggering system shown in FIG. 2—for the sake of simplicity, only one measured signal is used for triggering in the triggering system of FIG. 2—compares the measured signal present at the respective input A, and pre-amplified with reference to its amplitude, in each case with reference to overshooting or undershooting the level of a threshold-value signal present at the input B, in order to form complex triggering conditions. The respective, variable threshold values 1 . . . N are stored in digitized form in a register. A triggering signal for the correct presentation of the required signal portion of the measured signal in the recording unit is generated in an evaluation unit connected downstream of the comparators dependent upon the respectively-set triggering condition and subject to the occurrence of the triggering condition.
If the intersection of the threshold-value signal with the measured signal to be recorded—triggering point—is disposed between two sampled values of the digitized, measured signal to be recorded, a triggering offset is provided, as shown in FIG. 3, between the triggering point and the next sampled value or respectively between the triggering point and the preceding sampled value of the digitized, measured signal to be recorded. This triggering offset leads to a phase offset in the presentation of the measured signal in the recording unit between the measured signal to be recorded and the coordinate origin of the recording unit.
This phase offset—jitter—between the measured signal to be recorded and the coordinate origin of the recording unit is also known in analog oscilloscopes and is presented in FIG. 4. In FIG. 4, a jitter of 0.3 display units on the screen of the oscilloscope is evident for a triggering threshold of 0 V.
It is known from analog oscilloscopes that this jitter adopts either a constant value—systematic jitter —, if the distance between triggering point and the next sampled value of the digitized measured signal is largely unchanged under stable conditions, or a statistical value—statistical jitter—for example, if stochastic phase noise is superposed on the measured signal.
In addition to these phase errors in the oscilloscope display resulting from time discretisation of the measured signal and stochastic phase noise of the measured signal, undesired jitter can also result from different delay times in the measurement channels of measured signals, which are used in combination for checking the triggering condition. In this context, it is also a disadvantage that the triggering condition may not occur at all, and accordingly, the measured signals are not even presented on the oscilloscope screen.
The fact that phase and respectively time errors in the two paths of the measured signal—analog-digital converter or triggering system—are mutually superposed and under extreme conditions can bring about a doubling of the effect can be seen as an additional disadvantage.
DE 39 36 932 A1 discloses a triggering-signal generator for an oscilloscope, which generates the triggering signal in a digital manner.